VESTIGIAL WING CLAWS ON GREAT GRAY OWLS, STRIX-NEBULOSA Canadian field-naturalist [0008-3550] NERO (1984) volume: 98 issue: 1 page: 45-46

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Notes

Vestigial Wing Claws on Great Gray Owls, Strix nehulosa

Robert W. Nero 1 and Steven L. Loch-

1 Manitoba Wildlife Branch, 1495 St. James Street, Winnipeg, Manitoba R3H 0W9
-Department of Biological Sciences, St. Cloud State University, St, Cloud, Minnesota 56301

Nero, Robert W., and Steven L. Loch, 1984. Vestigia! wing claws on Great Gray Owls, Strix nehulosa, Canadian Field-
Naturalist 98(f): 45 46.

Great Gray Owls, Strix nehulosa, were found to have vestigial wing claws on digits El and 111. both as juveniles and adults.
Small claws on juveniles are common but often disappear by adulthood. Claws are present in some adults, in a few cases
reaching lengths exceeded only by the wing claws of New World condors (Gynmogyps, Vultur). The claws have no apparent
function.

Key Words: Great Gray Owl, Strix nehulosa, vestigial wing claws.

Wing claws on birds are vestigial and non-
functional; the Hoatzin {Opisthocomus cristatus)
(Pycraft 1903) and African Finfoot {Podica senega-
lensis) (Percy 1963). which use the wing claws for
climbing, are exceptions. The occurrence of wing
claws was studied by Fisher (1940) who examined
2004 specimens of 227 (mostly North American) gen-
era, He found that alular claws were rather common,
occurring in 1 4 of 2 1 orders examined. Claws on digit
III were less common and occurred only in birds
which also possessed alular claws. Such claws were
found frequently in natal Anseriformes, and were
present but uncommon in four other orders. Fisher
found small {C L5 mm) alular claws on two of four
strigiform genera, and daws on digit III on natal Otus\
but he did not examine owls of the genus Strix, The
occurrence of wing claws on Great Gray Owls [Strix
nehulosa) is reported here.

The senior author first observed alular claws on
Great Gray Owls while examining an injured owl in
September 1 979 (Figure i ). During the next four win-
ters about three-quarters of 109 owls handled for
banding and approximately 20 dead owls were exam-
ined for alular claws. Claws were found by probing the
distal region of the alular phalanx. Of approximately
50 adults examined, 9 (18%) possessed claws on the
alula, and 6 of 35 immatures (17%) possessed such
claws. Claws on adults were less than i mm in diame-
ter and ranged in length from 4 to 16.5 mm (x = 9.5,
n = 13). On any one bird (with one exception), length
varied only slightly « 25%). The claws (n = 12) on
immatures were all less than 5 mm in length. The
frequency represents minimum occurrence as some
claws likely were overlooked. This was especially so
for immatures as their claws were shorter. Also, claws
found were rigid and firmly attached; those not having
such characteristics likely were missed.

FIGURE L Great Gray Owl wing claw; 16.5 mm in length
(chord). This is the largest claw thus far observed in
this species. Note that it is atypical, i.e., twisted rather
than curved.

Here we refer to digits 11 and II rather than digits I
and II. Independent studies by IVlontagna ( 1945) and
Holmgren (1955) indicated that the thumb and little
finger were lost in birds, and that phalanges of digits
II, III, and IV remain (Marshall 1960).

Claws on digit III were usually not looked for; 3
dead immature owls were carefully examined and
minute claws were found on that digit. Nestlings were
examined for wing claw r s in the summer of 1980.
Claws were observed by parting and sometimes wet-
ting the feathers at the distal ends of digits II and III of
one wing. Eight of 8 and 6 of 9 bore claws on digit II
(100%) and digit IN (67%) respectively. All claws were
less than 4 mm in length,

These data suggest that most Great Gray Owls bear
wing claws when young, but many are lost, for reasons
unknown, as the owl matures. This is possibly a con-
sequence of the degenerative nature of the claw-
producing tissues. Also, claws may be lost owing to
abrasion and other factors. In some birds the alular
claws persist and grow and may become relatively
large. In Fisher's ( 1940) report, only the claws of New.

45

46

The Canadian Field-Naturalist

Vol. 98

World condors {Gymnogyps, Vuhur) exceeded the
greatest lengths reported here.

Thirteen aiular claws removed from adults pro-
vided a basis for their description. The claws were
embedded in the flesh on the dorsal surface of the
distal extremity of the phalanx. The claws appeared
not to articulate with or be reinforced by the phalanx,
in contrast to the situation with normal claws (on feet
of birds). Likewise, claws were embedded in the flesh
at the terminal phalanx of digit I'll. Claws usually
grew parallel to adjacent feathers and curved toward
the ventral surface of the wing. Some claws, especially
larger ones, tended to project anteriorly and to twist
rather than curve.

The claws consisted of a hard, dense (though partly
hollow) keratinized body covered in part or more
often in total with a layered coating of soft, white,
opaque material ( Figure 2), The claws of nestlings and
of most adults were entirely covered, Only remnants
of the coating were adhering to the base of two other
claws, and in three claws the coating was entirely
lacking, apparently the result of abrasion. The coating
appeared as a series of numerous overlapping layers
which were easily removed, breaking off in flakes
which readily crumbled to powder.

With the coating removed, the horny surface of the
claw appeared a translucent blackish or gray. Fine
parallel striations encircled the claw, and occasional
abrupt changes in diameter caused the claw to appear
pinched or restricted; those aspects were apparently
growth-related.

The base of each claw was hollow and appeared as a
membranous papery tube. The mid-section or stem of
the claw had a round or more often flattened, some-
times slightly indented, ovoid cross-section. The tip of
the claw was usually pointed and roughly triangular in
cross-section. In two claws the tip was more complex
than in the others, consisting of a central pointed
portion with an attached piece on both sides.

Under magnification the claw appeared as a deli-
cate tubular structure. In one claw a minute duct
coursed its entire length; likewise, in another a ropy
structure existed. In one claw there was an obvious
pointed tip and tubular body protruding from inside
another, not unlike the two claws described above.
Somewhat similarly, a series of pointed tubes within
tubes was observed when wing claws from a Red-
winged Blackbird were dissected (Nero 1957),

Although perhaps only a trivial feature, the pres-
ence of wing claws in Great Gray Owls raises some

Figure 2. Great Gray Owl wing claw; 11.0 mm in length
(chord).

fundamental evolutionary questions. Presumably the
presence of these atavistic wing claws has caused no
particular selective disadvantage. Perhaps this may be
the explanation for their persistence over the great
period of time since birds evolved from reptiles.

Acknowledgments

Various reviewers deserve credit for advice and
recommendations for improving this paper, particu-
larly John C. Barlow, Peter Stettenheim and one
unknown reviewer. Chris McGowan and Robert J.
Raikow referred us to some literature, Allan Peden
drove a long way to deliver the owl with the large wing
claw r that elicited the senior author's interest in this
subject. Robert R. Taylor kindly photographed the
two wing claws. Herbert W. R. Copland has been a
constant field colleague to the senior author and
deserves thanks for assistance is examining live and
dead Great Gray Owls.

Literature Cited

Fisher, H. I. 1940. The occurrence of vestigial claws on the
wings of birds. American Midland Naturalist 23: 234-243,

Holmgren, N. 1955. Studies on the phylogeny of birds.
Acta Zoologica (Stockholm) 36: 243-328.

Marshall, A. J. 1960. Biology and comparative physiology
of birds, Volume I, Academic Press, New York.

Montagna, W. 1945. A re-investigation of the development
of the wing of the fowl. Journal of Morphology 76:
87-113.

Nero, R. W, 1957, Vestigial claws on the wings of a Red-
winged Blackbird. Auk 74: 262.

Percy, W, 1963. Further notes on the African Finfoot, Pod-
ka senegalemis (VieiUot). British Ornithologist's Club
Bulletin 83: 127-132.

Pycraft, W. P, 1903, The claws on the wings of birds.
Knowledge 26: 221-224.

Received 2 July 1982
Accepted 14 September 1983